Recently, digital still cameras (hereinafter referred to simply as digital cameras) and portable terminal devices, such as portable telephones, that include a camera have become very popular. These cameras convert an image obtained through imaging optics to an electrical signal using an image pickup element, such as a CCD (charge-coupled device) or a CMOS (complementary metal-oxide-semiconductor), and record the signal as image data. In such cameras, a small wide-angle imaging lens is required. In particular, many two-component, two-element imaging lenses have been proposed based on their low cost and small size. For example, Japanese Laid-Open Patent Applications H06-067089, H11-295592, and 2001-183578 disclose such imaging lenses.
Japanese Laid-Open Patent Application H06-067089 disclose imaging lenses that include a first lens element made of plastic (i.e., synthetic resin) having negative refractive power and a meniscus shape with its convex lens surface on the object side, and a second lens element that is a biconvex lens component made of plastic with its object-side lens surface being spherical and having a large radius of curvature. These imaging lenses enable favorable correction of various aberrations while having the imaging lenses be short and compact.
Japanese Laid-Open Patent Application H11-295592 also discloses imaging lenses that include two lens elements. The first lens element is made of plastic, has negative refractive power, and has a meniscus shape with its convex lens surface on the object side. The second lens element is a biconvex lens component with nearly the same curvature on both lens surfaces.
Japanese Laid-Open Patent Application 2001-183578 also discloses an imaging lens that includes two lens elements with two or more aspheric lens surfaces, including an aspheric lens surface on each of the lens elements, and that satisfies specified conditions of lens parameters. This imaging lens is constructed with a small number of lens elements, is inexpensive, and is designed for use with a camera that uses a CCD or a CMOS as an image pickup element.
However, the imaging lenses of Japanese Laid-Open Patent Application H06-067089 have a half-field angle as small as 18.4° which thus does not provide a sufficient field angle (i.e. field of view). In the imaging lenses disclosed in Japanese Laid-Open Patent Application H11-295592, the ratio of the distance from the object side lens surface of the imaging lens to the image plane divided by the focal length of the imaging lens is in the range 3.8–4.5, and in Japanese Laid-Open Patent Application 2001-183578, the ratio of these distances is 2.45. The total length of the imaging lenses becomes relatively long in both cases, leaving much room for improvement in compactness of the imaging lenses.